In many utility vehicles, such as agricultural vehicles and harvesting machines, hydraulic drives are applied. They include a pump driven by an internal combustion engine and a motor connected to the pump so as to conduct hydraulic fluid, that drives a wheel or several wheels. In some vehicles the wheels of the front axle and the rear axle are driven hydraulically in each case by at least one motor associated with the axles.
In such hydraulic all-wheel drives there is a problem in certain operating situations, particularly in operating downhill, when a greater load is applied to the front axle than to the rear axle. The drive motors then operate as pumps that generate a pressure that may be greater than the pressure of the pump. Due to the differing friction forces at the wheels acting on the ground due to the differing loads, the condition can occur that the direction of the flow at an axle carrying a smaller load reverses and that the wheels on this axle rotate opposite to the direction of operation. This results in the so-called back-spin effect.
U.S. Pat. No. 5,199,525 A describes a control circuit for a hydraulic all-wheel drive. A hydraulic motor is associated with each of the front and rear axles that are driven by a single pump. A pressure controlled valve arrangement is provided, between the pump and the motor at the rear wheel, that is critical relative to the back-spin effect. When a pressure difference in the supply line of the motor of the rear wheel reaches a pre-determined value, the valve changes its position. In this position the pressure load for the motor of the rear wheel is interrupted by the pump arrangement and is now operating in a no-load condition. Thereby the occurrence of the back-spin effect is reduced, but at the expense of the braking effect of the rear wheel, which in a corresponding situation, only runs along without any load.
DE 199 18 882 A describes a hydraulically driven vehicle, particularly a harvesting machine, where the individual wheels are associated with hydraulic motors and rotational speed sensors. Calculated rotational speeds are compared with measured rotational speeds. Each rotational speed signal that exceeds the calculated value indicates a slipping or overrunning wheel. A signal is sent to the displacement control plate of the particular motor in order to reduce its fluid displacement and to avoid an overrunning of that wheel.
Operating situations are conceivable in which a single sensor detects either the rotational speed of a wheel or the pressure difference in the supply line of a hydraulic motor, and delivers a measured value, that does not permit any useful conclusion as to the operating situation and the measures that are to be taken. In that way, the pressure difference can point to a motor that is rotating, but whose driven wheel is slipping relative to the ground. On the other hand, a rotational speed sensor also can point to a rotating wheel even though it is subject to the back-spin effect.
The problem underlying the invention is seen in the need to make available an improved drive system for a utility vehicle that does not suffer the aforementioned disadvantages.